Denitrification in groundwater of aquifers is an important process that helps to maintain environmental standards, yet there is a limited number of studies that determine the spatial variation of denitrification conditions in aquifers on a regional scale. This paper presents a procedure for the regionally differentiated derivation of denitrification conditions in groundwater based on measured values of the redox-sensitive parameters oxygen, iron, manganese, DOC and nitrate as well as information on aquifer typologies. We applied this procedure to Germany, using measured values from more than 24,000 groundwater monitoring sites from 2007-2016. Annual concentrations of the five parameters at the monitoring sites were regionalized using an optimized, iterative inverse distance weighting procedure, using 15 aquifer typologies for spatial delineation. The annual grids (2007-2016) of each parameter were then overlaid and a median over time was calculated. Discrete ranks were then assigned to the concentrations of each parameter depending on their redox class, and ultimately, after overlaying the five parameters, a mean value was calculated describing the nitrate degradation capacity in groundwater.To assess the plausibility of the derived denitrification conditions in groundwater, the latter were linked to reaction constants of denitrification in groundwater that assume a dependence of the extend of denitrification on the travel time in groundwater. A comparison of the modeled quantities of denitrified nitrate with the proportion of denitrified nitrate determined with the N 2 /Ar method at 820 groundwater monitoring wells in three German Federal States showed good agreement. Accordingly, the method presented here is suitable to be used consistently for larger regions for the regionally differentiated derivation of denitrification conditions in groundwater. For regions with denitrifying groundwater conditions, the results provide an explanation for the frequently observed discrepancy between high nitrate emissions from soil and low nitrate concentrations in the groundwater of intensively used agricultural areas.

Denitrification in groundwater is an important process that helps to maintain environmental standards, yet there are very few studies that determine the spatial variation of denitrification conditions in aquifers on a regional scale. We introduce a procedure to derive spatially continuous estimates of denitrification conditions in groundwater based on the interpolation of measurements of the redox-sensitive parameters oxygen, nitrate, iron, manganese and DOC, combined with the quantification of denitrification using a 2D-hydrodynamic model based on first-order reaction kinetics. We applied this procedure to Germany, using measured values from more than 24,000 groundwater monitoring sites from 2007 to 2016. Annual concentrations of the five parameters at the monitoring sites were regionalized using an optimized, iterative inverse distance weighting procedure within 15 aquifer typologies for spatial delineation. The annual grids (2007–2016) of each parameter were then overlaid and a median over time was calculated. Discrete ranks were then assigned to the concentrations of each parameter based on their redox class, and ultimately, after overlaying the five parameters, a mean value was calculated describing the nitrate degradation conditions in groundwater. After assigning half-life times and reaction constants to those denitrification conditions, we quantified denitrification in groundwater using the hydrodynamic model WEKU.To assess the plausibility of the derived denitrification in groundwater, we compared our results with the proportion of denitrified nitrate determined with the N2/Ar method at 820 groundwater monitoring wells in three German Federal States, which showed an overall good agreement. Accordingly, the method presented here is suitable to be used for the regionally differentiated derivation of denitrification conditions in groundwater. For regions with denitrifying groundwater conditions, the results provide an explanation for frequently observed discrepancies between high nitrate emissions from the soil and low nitrate concentrations in the groundwater of intensively used agricultural areas.

In Germany, modelled nitrate concentrations in the leachate are of great importance for the development of scenarios for the long-term achievement of the groundwater quality target according to the specific requirements of the EU Water Framework Directive as well as within the context of the recently adopted general administrative regulation for the designation of nitrate-polluted areas in Germany. For the German federal states of North Rhine-Westphalia (NRW) and Rhineland-Palatinate (RLP), an area-covering modelling of mean long-term nitrate concentrations in leachate with high spatial resolution was carried out using the model system RAUMIS-mGROWA-DENUZ. Hotspot regions with nitrate concentrations in the leachate of 50 mg NO3/L and more were identified for intensively farmed areas in the Münsterland, Lower Rhine, and Vorderpfalz. The validity of modelled values was checked using measured values from 1119 preselected monitoring stations from shallow springs and aquifers filtered near to the surface with oxidizing properties. For the land use categories of urban areas, arable land, grassland, and forest, an at least good agreement of modelled nitrate concentrations in the leachate and measured nitrate concentrations in groundwater was obtained at numerous sites. An equally good agreement was obtained for 1461 measuring stations from the area of responsibility of the Erftverband, which is a major water supplier in the Lower Rhine region. Here, discrepancies have been analyzed in detail due to profound regional knowledge on observation sites. It turned out that in most cases, accuracy limitations of input data (e.g., N balance surpluses of agriculture at the municipal level, 1:50,000 soil map) have been the reason for larger deviations between observed and modelled values. In a broader sense, the case study has shown on the one hand that the model system RAUMIS-mGROWA-DENUZ is able to reliably represent interrelationships and influencing factors that determine simulated nitrate concentrations in the leachate. On the other hand, it has been proven that observed nitrate concentrations in groundwater may provide a solid data source for checking the plausibility of modelled nitrate concentrations in leachate in cases where certain preselection criteria are applied.

In order to analyze the impact of climate change on groundwater resources in North Rhine-Westphalia a multi-model ensemble for projecting future groundwater recharge was established. The ensemble consists of 36 members of the model chain RCP-GCM-RCM-mGROWA in total, i.e. combinations of 3 greenhouse gas concentration trajectories, 6 global and 5 regional climate models, and the water balance model mGROWA. The ensemble projections show only a few significant changes of groundwater recharge in the future periods 2011-2040, 2041-2070, and 2071-2100. A robustness test using the two-criteria model agreement and the significance of the individual model projections did not reveal systematic and significant changes of groundwater recharge until 2100. From the statistical point of view, groundwater recharge can be expected to remain at the current level. Hydrometeorological, North Rhine-Westphalia is located in a transition zone in which the impact of the rising winter precipitation on groundwater recharge is counter-balanced by the impact of warming.

Zusammenfassung Mit einem Multi-Modell-Ensemble wurde analysiert, wie sich der Klimawandel auf den Grundwasserhaushalt in Nordrhein-Westfalen (NRW) auswirkt. Hierzu wurden Projektionen der zukünftigen Grundwasserneubildung für insgesamt 36 Mitglieder der Modellkette RCP-GCM-RCM-mGROWA, bestehend aus 3 RCP-Szenarien zukünftiger globaler Erwärmung, 6 globalen und 5 dynamischen regionalen Klimamodellen sowie dem Wasserhaushaltsmodell mGROWA, vorgenommen. Mit dem Ensemble wurden für die hydrogeologischen Großräume NRWs nur teilweise signifikante Änderungen der jährlichen Grundwasserneubildung in den Perioden 2011–2040, 2041–2070 und 2071–2100 projiziert. Ein Robustheitstest mit zwei Kriterien (Übereinstimmung und Signifikanz der Änderungssignale) liefert keine belastbare Begründung dafür, dass sich die Grundwasserneubildung bis 2100 systematisch und signifikant ändern wird. Aus statistischer Perspektive wird deshalb die Schlussfolgerung gezogen, dass in NRW langfristig eine Grundwasserneubildung erwartet werden kann, die sich nicht grundlegend vom Niveau der Periode 1971–2000 unterscheidet. Hydro-meteorologisch befindet sich NRW in einer Übergangszone, in der eine Zunahme der Winterniederschläge die Wirkung der Erwärmung auf die Grundwasserneubildung wahrscheinlich kompensiert.

Reaching the EU quality standard for nitrate (50 mg NO3/L) in all groundwater bodies is a challenge in the Federal State of North Rhine-Westfalia (Germany). In the research project GROWA+ NRW 2021 initiated by the Federal States’ Ministry for Environment, Agriculture, Nature and Consumer Protection, amongst other aspects, a model-based analysis of agricultural nitrogen inputs into groundwater and nitrate concentration in the leachate was carried out. For this purpose, the water balance model mGROWA, the agro-economic model RAUMIS, and the reactive N transport model DENUZ were coupled and applied consistently across the whole territory of North Rhine-Westfalia with a spatial resolution of 100 m × 100 m. Besides agricultural N emissions, N emissions from small sewage plants, urban systems, and NOx deposition were also included in the model analysis. The comparisons of the modelled nitrate concentrations in the leachate of different land use influences with observed nitrate concentrations in groundwater were shown to have a good correspondence with regard to the concentration levels across all regions and different land-uses in North Rhine-Westphalia. On the level of ground water bodies (according to EU ground water directive) N emissions exclusively from agriculture led to failure of the good chemical state. This result will support the selection and the adequate dimensioning of regionally adapted agricultural N reduction measures.

Excessive nitrate inputs into groundwater have been recognized as a main reason for failing drinking water standards since decades. Agricultural N-emissions originating from mineral or organic fertilizers are regarded as the most relevant source of nitrate in groundwater worldwide. Accordingly, strategies to cope with the nitrate pollution of groundwater are focused on controlling the agricultural sources of nitrate. In Europe this is reflected in the water legislation on EU level, i.e. the EU Water Framework Directive (EU-WFD), the EU Marine Strategy Framework Directive and the EU Nitrates Directive, obliging the polluter to implement measures to reduce the nitrogen impact on groundwater.With an average population density of 525 inhabitants/km2 the Federal State of North Rhine-Westphalia represents an example for a densely populated region in Germany. Consequently, the assessment of water bodies showed that a number of groundwater and surface water bodies are not in good status due to high nitrogen loads resulting e.g. in high nitrate concentrations in groundwater. There is a debate in North Rhine-Westphalia to what extent agricultural and non-agricultural N-emissions contribute to high nitrate concentrations.The German Working Group on water issues of the Federal States and the Federal Government, require that the nitrate concentration in the leachate should not exceed 50 mg NO3/l. Against this background it is obvious that the nitrate concentration in the leachate represents a decisive parameter for both, the assessment on the nitrate pollution of groundwater and as starting point to determine the N reduction requirements.We used an interdisciplinary model network consisting of a nutrient balance model, a nutrient balancing model (RAUMIS, Henrichsmeyer et al., 1996), a water balance model (mGROWA, Hermann et al., 2015), a reactive nitrate transport model in soil (DENUZ, Wendland et al., 2009) and a reactive nitrate transport model in groundwater (WEKU, Kunkel & Wendland, 1997) to predict the nitrogen intakes and the nitrogen losses to groundwater and surface waters from different input sources and pathways.The nitrogen flux was modelled using nitrogen input data from the time period 2014-2016 and hydrological data for the time period 1981-2010. The nitrate concentrations in the leachate were calculated separately for agricultural and non agricultural N-sources involved, to enable the identification of the main polluter in a certain region, i.e. the one who has to implement measures to to reduce the nitrogen impact on groundwater.From the model analysis it becomes evident that non-agricultural sources do only locally cause nitrate concentrations in the leachate above 50 mg NO3/l in spite of the high population density (525 inhabitants / km2). It could be confirmed that agricultural sources (N-balance surpluses from agriculture and atmospheric NH4 deposition) are exclusively responsible for extended areas of nitrate concentrations above 50 mg NO3/l. Especially in the northern (Münsterland) and western (Lower Rhine basin) parts of the Federal State the implementation of measures to reduce agricultural N-emissions in the context of the WFD program of measures is necessary. These results will not only support the right dimensioning of agricultural N-reduction measures, but also affect the selection and implementation of regionally adapted N-reduction measures.

For the Federal State of Lower Saxony, Germany, nitrogen management options are developed and implemented in three pilot areas using new participation approaches and technologies suitable for programs of measures to reduce diffuse pollution from agriculture. As a target value for water protection measures a nitrate concentration in percolation water of 50 mg/l as an average for a larger area defined by the groundwater bodies and their hydrogeological subdivisions has been defined. An integrative emission model is used to simulate the interactions between agricultural practice, nitrogen surpluses and the nitrogen flow through the soil and aquifer to the outflow into surface waters. The actual nitrate concentrations in percolation water are calculated for the entire Federal State of Lower Saxony considering site-characteristics, N-surpluses, water balance and denitrification in the soil. The tolerable N-surpluses needed to meet the environmental target are quantified as averages for each of the hydrogeological subdivisions by "backward" calculation using this model system. The required reduction of N-surpluses was estimated by comparing the tolerable N-surpluses with the actual state of nitrogen emission. For the evaluation of the amount and efficiency of water protection measures, the required reduction of N-surpluses to accomplish the environmental target is quantified, using the current status as a reference.